Description

This function works for any stream that supports non-blocking mode
(currently, regular files and socket streams).

Parameters

stream

The stream.

mode

If mode is FALSE, the given stream
will be switched to non-blocking mode, and if TRUE, it
will be switched to blocking mode. This affects calls like
fgets() and fread()
that read from the stream. In non-blocking mode an
fgets() call will always return right away
while in blocking mode it will wait for data to become available
on the stream.

When you use fwrite() on a non-blocking stream, data isn't discarded silently as t dot starling said.

Remember that fwrite() returns an int, and this int represents the amount of data really written to the stream. So, if you see that fwrite() returns less than the amount of written data, it means you'll have to call fwrite() again in the future to write the remaining amount of data.

You can use stream_select() to wait for the stream to be available for writing, then continue writing data to the stream.

Non-blocking streams are useful as you can have more than one non-blocking stream, and wait for them to be available for writing.